Patch module manager

Extract the "load" function from main.js and create a module named moduleManager able to load and unload modules on-the-fly.

### ModuleManager

Useful for operating modifications on a module without restarting NexBot.
This module allow to load and unload modules on-the-fly.
If a module is already loaded, then the cache will be cleared and the module will be reloaded.
This module itself can be reloaded.


### Restrictions
Administrators only.

### Usage
```/msg NexBot load <Module>``` - Load module
```/msg NexBot unload <Module>``` - Unload module